In general, a semiconductor radiation detector element is a device in which an electric charge produced therein due to ionization effect of incident radiation is swept and collected to produce a signal under an electric field applied between both electrodes. Therefore, it is very important to collect the electric charge with higher efficiency to attain higher resolution for energy relative to the incident radiation.
In order to get such higher efficiency for collection of the charge it is preferable that the distance “l” for migration of charge carriers (electron and hole) is longer, or in other words, the strength of electric field is stronger. The distance “l” for migration of charge carriers is defined as follows:l=μτEwhere μ is mobility of an electric charge; τ is life of a carrier; and E is strength of an electric field.
On the other hand, all the semiconductor radiation detector elements have such tendency that some leakage current constantly flows according to the voltage applied even if there is no incident radiation present, which is one of the causes that lowers the resolution for energy. As the result, the voltage that may be applied across both electrodes is limited.
A “P” type CdTe crystal for a radiation detector element has electric specific resistance in the order of ˜109 Ωcm. The crystal has been used to fabricate a semiconductor radiation detector element of MSM (metal electrode—semiconductor—metal electrode) type in such manner that ohmic electrodes are formed on opposite surfaces of the crystal. However, such device is defective in that it has insufficient capability of suppression of leakage current if sufficient bias voltage is applied to get good collection of the charge. Inversely, if reduced bias voltage is applied to decrease the leakage current, collection of the charge becomes insufficient. Therefore, no satisfactory resolution for energy can be attained in the device of MSM type.
In order to overcome the deficiency of CdTe semiconductor radiation detector element of MSM type, as described above, a semiconductor radiation detector element of Schottky barrier type has been proposed in which an electrode of indium, etc. is provided on one surface of “P” type CdTe crystal to form a Schottky contact therebetween and an electrode of gold, platinum, etc. is provided on opposite surface of the crystal to form an ohmic contact therebetween.
The semiconductor radiation detector element of Schottky barrier type, as described above, has been found to provide good rectifying characteristic so that leakage current, if any, can be suppressed to the minimum even in the presence of higher electric field by applying some reversed voltage, thereby providing good resolution for energy.
In the CdTe semiconductor radiation detector element having metal material, e.g. indium, as the electrode at the side of Schottky barrier, however, such phenomenon has been observed that the efficiency of charge collection becomes significantly reduced with the time after application of bias voltage. This phenomenon is referred to as “Polarization Effect”. The phenomenon has been considered to be caused due to the fact that because of incomplete of Schottky contact at positive side and ohmic contact at negative side there is any distortion present in band structure, and accordingly, a hole is restrained in a filled band to form an electron trapping center before the hole reaches the negative pole. That is to say, in the CdTe semiconductor radiation detector element of Schottky barrier type having indium used as the electrode, good resolution for energy is provided immediately after application of the voltage, but it lowers with the progress of time, which has imposed significant problem for actual usage.
In view of the above, it is an object of the present invention to solve the problems for prior art electrode configuration, as described above, and to provide an improved radiation detector element of Schottky barrier type that is unlikely to have “Polarization Effect” by incorporation of improved electrode configuration.